Re: [Discuss-gnuradio] Problems when receiving after transmit

[Dan Halperin]

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Interesting. I don't have a logic analyzer handy but after the holiday I
may find someone who knows how to use one. More importantly, do you have
suspicions about the solution? ;)

As another data point, I've found that performing the second receive at
different frequencies can bring the noise floor back to normal. If I
transmit at 2.48G and receive at 2.485G the noise floor is normal. With
some other RX frequencies, (I think 2.445G) the same problem persists.

- -Dan

Johnathan Corgan wrote:
> Dan Halperin wrote:
>> Just-powered on USRP rev 4.2 with RFX2400 rev 2-6-2006. Current SVN.
>>
>> usrp_rx_cfile.py -f 2.485G -d 8 -s pre.dat
>> usrp_siggen.py -i 16 -f 2.485G
>> usrp_rx_cfile.py -f 2.485G -d 8 -s post.dat
>>
>> In the first set of data, the noise level is what I'd expect and
>> everything works hunky-dory. After the siggen, however, it all goes
>> haywire, the noise floor becomes huge even though an adjacent USRP sees
>> nothing.
> 
> I wonder if somehow the USRP transmitter is getting left powered up.
> The mixer would be getting random noise and upconverting it to your
> passband.  This would feed through from the blocked side of the TX/RX
> switch (about 30dB of attenuation) into the receiver.
> 
> You'd have to measure the logic level at the mixer chip enable pin to be
> sure.
> 
> If this is the case, I have a suspicion about the cause.
> 

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